PTFE Rods, Bushes, Sheets, Machined Components

Mechanical Properties of PTFE

Deformation under Load (Creep) & Cold Flow

This property is an important consideration in the design of parts from polytetrafluoroethylene. PTFE deforms substantially over time when it is subjected to load. Metals similarly deform at elevated temperatures. Creep is defined as the total deformation under stress after a period of time, beyond the instantaneous deformation upon load application. Significant variables that affect creep are load, time under load, & temperature.

Resin manufacturers have long recognized the excessive deformation of polytetrafluoroethylene in applications where parts such as gaskets & seals experience high pressures. Copolymers of tetraethylene with small amounts of other fluorinated monomer are known as Modified PTFE resins & have been reported to exhibit reduced deformation under load.

Fatigue Properties

Flexibility characteristics are of paramount importance in many applications involving motion. A valve diaphragm is a good example of a part where a polymer membrane experiences repeated movement. Flex life is defined as the number of cycles that a part can endure before catastrophic fatigue occurs; the higher the molecular weight, the higher the flex life. Crystallinity has a detrimental effect on flex life; the higher the crystallinity, the lower the flex life.

Impact Strength

Impact strength of a part depends on its ability to develop an internal force multiplied by the deformation of the part as a result of impact. The shape of a part, such as a metal spring as opposed to a flat metal plate, can Enhance its ability to absorb impact PTFE resins have excellent impact strength in a broad temperature range.

Hardness

Hardness of PTFE is determined by a number of methods, such as ASTM D758 or D2240 (Rockwell R Scale), or by Durameter scales.

Fillers improve the hardness of PTFE by 10% – 15%, which is preserved over a wide range of temperatures. Increasing the filler content, in general, elevates the hardness of the compound.

Abrasion and Wear

Polytetrafluoroetylene parts have good wear properties. The resistance of unfilled PTFE to wear is less than that of filled compositions.

Electrical Properties

Electrical stability of polytetrafluoroethylene is outstanding over a wide range of frequency and environmental conditions. This plastic makes an excellent electrical insulator at normal operating temperatures. Dissipation factor and virtually constant up to 10 MHz Dielectric strength of PTFE drops off with increasing frequency slower than most other material.

Thermal Behavior of PTFE

Polytetrafluoroethylene resins are very stable their normal use temperature range (< 260 0C). They exhibit a small degree of degradation at higher temperatures.

Thermal Expansion

A polytetrafluoroethylene part contracts 2% when it is cooled from 23 0C to -196 0C & expand 4% upon heating from 23 0C to 249 0C. These dimensional charges are significant to the design, fabrication, & use of PTFE parts.

Thermal Conductivity & Heat Capacity

Polytetrafluoroethylene resins have very low thermal conductivity & are considered good insulators.

Irradiation Resistance of PTFE

Polytetrafluoroethylene & other perfluorinated fluoropolymers are quite susceptible to radiation. Exposure to high energy radiation such as X-rays, gamma rays, & electron beams, degrades PTFE by breaking down the molecules & reducing its molecular weight. As in thermal degradation, radiation stability of PTFE is much better under vacuum compared to air.

Typical Properties of Filled Fluoro polymers

Mechanical Properties

Polytetrafluoroethylene retains excellent properties at very low and high temperatures.

Table 3.11 provides summery of some of the mechanical properties of three different compound containing 65% bronze, 15% carbon, & 25% glass fiber at different temperatures. Properties of unfilled PTFE have been listed for comparison. Tensile strength and break elongation at elevated temperatures are given in Table 3.12. All the listed compounds retain excellent tensile properties at above room temperature.

Deformation under load of all filled polytetraflouroethylene compounds decreases in comparison to unfilled resin, as seen in Table 3.13.

Combinations of carbon & graphite reduce deformation under load is bronze at 60% by weight. Hardness is increased by the addition of additives, particularly bronze, carbon & graphite Table 3.14.

Thermal Properties

Fillers reduce the liner coefficient of thermal expansion & contraction of compounds. Table 3.17 & Table 3.18 provide data for several compounds at different temperatures. Aluminum reduces the coefficient of thermal contraction the most due to its flat platelet structure; mica has a similar effect.

Chemical Property

Permeability of compound increase due to the voids. Polytetrafluoroethylene has excellent chemical resistance properties. The effect of incorporation of additives on chemical properties depends on the type of the filler & the specific chemicals. In general, chemical properties of filled PTFE compounds are not as good as those of the unfilled resin. Table 3.21 shows the effect of a number of chemicals on carbon/graphite, glass & bronze compound.

PTFE Skived Tape

Filled Compounds made With PTFE

ASTM D 4745. Describes the properties and characteristics parameters for filled molding compounds made with PTFE. The specification provides standards for bulk density, tensile strength and elongation of PTFE filled with different percentages of glass fiber, glass fiber and Molybdenum Disulfide, graphite, carbon and graphite, bronze, bronze and Molybdenum Disulfide and stainless steel.

PTFE Tubing

ASTM D 1710-08. The tubing is intended for electrical, mechanical, chemical and medical applications manufactured from extrusion resins made from PTFE resins.

Molding and Machining Tolerances for PTFE resin parts

ASTM 3297. This specification defines tolerances applicable to parts molded and free sintered from PTFE resins and to machine parts produced from basic shapes of compression molded or ram extruded resins. The thermal expansion of PTFE parts between 64F and 70F is non-uniform due to a critical transition zone characteristic of PTFE resins.

Standard grades of material of composition:

Virgin PTFE

Chemically Modified Virgin PTFE

15%-25% Glass Filled PTFE

5% / 15% Glass + 5% MOS2 Filled PTFE

25-30% Carbon Filled PTFE

15% Graphite Filled

40-60% Bronze Filled PTFE

55 % Bronze + 5 % MOS2 FILLED PTFE

Properties of Filled PTFE Compounds

Unfilled Polytetrafluoroethylene e is inadequate for number of mechanically demanding engineering applications. Cold Flow or creep would prevent the use of PTFE in Many mechanical Applications. Creep is defined as total deformation under stress after a period of time. Significant Variables that affect creep are load, time under load, and temperature.

Advantages of fillers have been found to improve a number of physical properties of PTFE, Particularly Creep and Wear rate.

Filled Granular Resins are found to be suitable for Gaskets, Shaft Seals, Bearing, and Bearing Pads. The choice and concentration of the filler depends on the desired properties of the final part. Glass Fiber, Bronze, Steel, Carbon, Carbon Fiber and graphite are among the common fillers materials.

Upto 40% by volume of filler can be added to the resin without complete loss of the physical properties. The Impact of the additives 5% below of filler on the properties of compound is insignificant. Above 40% most physical properties of the compounds drop sharply.

The only requirement for an additive to qualify as filler for PTFE is that it should be able to withstand the sintering temperature temperatures of PTFE. Sintering Involves exposure to temperature close to 400 Degree Celcius. Characteristics of the fillers such as particle size and shape affect the properties of the compound.Glass Fiber is the most common Filler with a positive impact on the creep performance of PTFE by reducing it at low and high temperatures. Wear characteristics of polytetrafluoroethylene are improved. Glass has little impact on the electrical properties of PTFE. Dielectric Breakdown Strength is somewhat adversely affected due to the increased porosity of parts. One drawback to the glass is the discoloration of sintered parts, more prevalent at higher temperatures.

Carbon reduces creep, increases hardness and elevates thermal conductivity of polytetrafluoroethylene. Wear resistance of the carbon filled components improves particularly when combined with graphite. Carbon filled Compounds performs well in non lubricated applications such as piston rings in compressor cylinders.Carbon Fiber lowers creep, increases flex and compressive modulus and raises hardness. These changes can be achieved in glass but less carbon fiber can achieve the same effects. Coefficient of thermal expansion is lowered and thermal conductivity is higher for compounds of carbon fiber PTFE. Graphite filled polytetrafluoroethylene has an extremely low coefficient of friction due to low friction characteristic of graphite. Graphite is chemically inert. Graphite imparts excellent wear properties of PTFE.

Bronze is the most popular metallic fibre.Large Quantity of (40-60% of the Weight). Large quantities of bronze reduce deformation under load and raise thermal and electrical conductivity of PTFE Compounds. These two characteristics are beneficial to applications where a part is subjected to load at extreme temperatures. Bronze is an alloy of Copper and tin and is attacked by acids and bases. It is oxidized and discolored during the sintering cycle with no impact on quality.

Molybdenum disulphide is an interesting additive. It increases the hardness of the surface while decreasing friction. Electrical Properties of the compound are virtually unaffected. It is normally used in small proportions combined with other fillers such as glass.

Reinforced Gasketing Material

Reinforced fine powder polytetrafluoroethylene material is primarily used for application as gaskets and seals in extreme temperature, pressure and chemical environments.

A Gasket in this type of application must be resilient and resistant to corrosive chemicals and also maintain high temperature and pressure. PTFE has necessary corrosion resistance to the majority of the industrial chemicals up to its melting point (327 Degree Celsius), but in its neat form (without fillers or additives) form it is not satisfactory in many applications because of the high cold flow (creep) that is inherent to PTFE.

After a short while an unfilled PTFE Gasket will begin to creep under pressure extended by the bolt loads that squeeze the gasket between the flanges. The net result of cold flow is loss of gasket thickness and leaks. An increase in temperature both accelerates and increases the creep.

The reinforcement approach deals with the problem of cold flow by highly filling PTFE with variety of fillers. Fillers are Hard Material such as metal powders, ceramic, glass fiber, carbon and others.

Fabrication of the reinforced gasket material is accomplished by filling the fine powder PTFE using the somewhat unusual process which incorporates the fillers in the polymer structure. Typically the sheets of the material are made of gaskets which can be stamped.